A large number of observations indicate the importance of macrophage Fc receptors in phagocytosis of opsinized particles and in antibody-dependent cellular cytotoxicity. Mouse macrophages have recently been shown to posess two Fc receptors differing in specificity and sensitivity to trypsinization. A panel of variants of the J774 macrophage cell line has been isolated which lacks the Fc receptor for IgG2b aggregates, but which binds IgG2a normally. The biochemical defect in the plasma membrane proteins of these variants will be investigated using electrophoretic and immunochemical techniques. Monoclonal anti-macrophage antibodies will be generated after immunization of rats with macrophage cell lines. These antibodies will be screened to ascertain if they inhibit Fc receptros. Selected antibodies will also be used to immunoprecipitate proteins on variants and parental J774 cells described above.